Support multiple deploy actions

[smoehrle]

Hey,

I started to implement a feature, where you can trigger multiple deployments at the same time (see example below). It's now possible to specify multiple cluster+service pairs which will be updated simultaneously. If this is a feature you are willing to merge, I'm happy to further clean up the code and fix/add some tests based on your comments.

Example:

# Previously, we had to start to individual deployments and wait for the first one to succeed to start the second.
ecs deploy app-messenger-cluster app-messenger-service --tag ${CI_COMMIT_SHORT_SHA} [...]
ecs deploy app-web-cluster app-web-service --tag ${CI_COMMIT_SHORT_SHA} [...]

# Now it's possible to start them at the same time:
ecs deploy app-messenger-cluster app-messenger-service app-web-cluster app-web-service --tag ${CI_COMMIT_SHORT_SHA} [...]

[trigrab]

I rebased the branch and added a test.